Conventionally, a composite camera such as a dome camera has been used for a surveillance camera constituting a monitoring system.
The composite camera uses a semiconductor image pick-up element (which will be hereinafter referred to as an image pick-up element) such as a CCD or a C-MOS.
In the image pick-up element, a sensitivity of an infrared region is much higher than a visible light level. In order to obtain a color image, therefore, an infrared cut filter is provided on the front surface of the image pick-up element to cut unnecessary infrared rays.
On the other hand, the surveillance camera needs to continue monitoring for twenty-four hours. Therefore, it is necessary to use a composite camera having a great dynamic range capable of clearly obtaining a bright image during the day and a dark image at night. In the case in which the composite camera using the image pick-up element is utilized for the surveillance camera, the following drawbacks are caused.
In the case in which the composite camera using the image pick-up element is utilized for the surveillance camera, it is suitable for a color image having a large information volume during the day. Moreover, if a sensitivity is set to obtain a clear image during the day, the sensitivity is reduced at night so that a clear image cannot be obtained.
For this reason, conventionally, the sensitivity has been set such that a clear image can be obtained at night, and a quantity of light incident on the image pick-up element has been decreased by regulating a diaphragm opening or a shutter speed, thereby adjusting the sensitivity of the image pick-up element during the day.
According to such a method, however, means for regulating a diaphragm or a shutter is required. Consequently, there has been a drawback in that the structure of the camera becomes complicated or expensive.
In order to eliminate the conventional drawbacks, the invention has been made and has an object to provide a camera and an optical filter switching method in which a clear image can always be obtained during the day and at night.